The present invention is an alternative to and/or improvement over multi-layer rigid-flex printed circuit boards disclosed, for example, in U.S. Pat. No. 4,800,461 issued Jan. 24, 1989 to Dixon, et al. under assignment to Teledyne Industries, Inc.
As noted in the above patent, techniques for making multi-layer rigid-flex PCBs are well known in the field as disclosed, for example, in U.S. Pat. No. 3,409,732, also assigned to Teledyne Industries, Inc. As disclosed in that earlier patent, rigid-flex stacked printed circuit boards include flexible printed circuit cables extending from the periphery of the rigid sections. The rigid portions of the flex cables are typically used as sites for electronic components or mechanical hardware. The copper conductors in each plane or layer are usually fabricated from one continuous sheet of copper foil.
With improvements in electronic technology, there has been a constant need for advances in electronic packaging. This need has led to more complex multilayer rigid-flex printed circuit boards with many boards now using up to twenty-five, or even more, layers of circuitry. However, severe problems developed when the rigid circuit portions included many layers of conductors and holes plated through with copper to provide conductor barrels connecting the conductor layers.
As also discussed in the background of U.S. Pat. No. 3,409,732, certain problems encountered in such rigid-flex printed circuit boards are created by the thermal expansion of typical insulator materials, such as acrylic adhesives and Kapton (a trademark of E.I. duPont de Nemours and Company, Inc. for polyimide film) utilized in the construction of rigid-flex PCBs. Thus, failures tend to occur when the board is subjected to elevated temperatures in thermal stress testing, hot oil solder reflow, and the like. The rate of thermal expansion (a fundamental material property) of the acrylic adhesive is about 30 percent, of Kapton about 10 percent, and of copper about 4 percent. When hot oil is used to reflow solder plated on the rigid printed circuit board, temperatures on the order of 450.degree.-500.degree. degrees F. cause expansion, for example, of the acrylic adhesive used to bond Kapton layers to copper layers in the multilayer rigid sections. As temperatures increase, the board, which is unrestrained, grows much faster in the thickness, or Z direction, then "copper barrels" formed in the plated through holes in the multilayer rigid board section. The copper barrels stretch as the acrylic adhesive and Kapton expand, sometimes fracturing the copper. Repeated cycles tend to break many of the plated copper barrels found in the holes in the rigid board sections.
If less acrylic adhesive is used to limit expansion, the internal stresses developed during lamination procedures cause unacceptable voids or delaminations in the final board. Since these deficiencies are not apparent until the final stages of construction, costly scrapping of nearly completed boards may be required.
It is now apparent that multilayer rigid-flex boards including insulator materials such as acrylic adhesive and Kapton always place Z-axis stress on plated through-holes. The coefficient of thermal expansion of the acrylic adhesive (Z-axis expansion) is the dominant influence. Because of the amount of acrylic required in many multilayer rigid-flex applications, all plated through-holes are stressed, with many of these cracking, making the boards unusable.
Another difficulty disclosed with the use of dielectric films such as Kapton in the rigid board area is their absorption of excessive moisture, on the order of up to 3 percent by weight of water. Absorbed moisture in the circuitry, which does not escape, may volatilize during high temperature operations and cause unacceptable delamination in the rigid board area. To remove the moisture from the Kapton and acrylic layers, the board must be baked at temperatures on the order of 250 degrees F. for many hours, for example 12, 24 or even 48 hours, an expensive process.
Yet another difficulty with insulator materials such as Kapton and acrylic adhesives involves the cleaning of holes, which have been drilled through the laminated rigid board, prior to plating through the holes. The excellent chemical resistance of the acrylic adhesive to typical cleaning solutions precludes the use of cleaning solutions for removal of smears resulting from hole drilling. Also, acrylics are prone to swelling due either to exposure to cleaning fluids during smear removal or to improper smear removal processing. To avoid these problems, expensive plasma etching is required to clean the holes.
As a solution to the above problems, the above noted Dixon, et al. patent provided a multilayer rigid-flex printed circuit board fabricated by a process providing a rigid section incorporating insulator materials which, when subjected to elevated temperatures, resisted expansion in the Z direction which would otherwise cause difficulties, including delamination and cracking of plated through copper barrels.
The Dixon, et al. process taught the formation of the flex sections, for example, from a prepreg glass layer laminated with Kapton insulation layers carrying acrylic adhesive as being sufficiently flexible and tear resistant to provide satisfactory results in use. The Kapton layers of the flex sections extended to but not substantially into, the rigid sections of the boards.
The Dixon, et al. patent also disclosed another embodiment wherein the flex sections of the rigid-flex printed circuit boards were fabricated without a prepreg glass layer to accord even greater flexibility therein.
The method and product disclosed by the Dixon, et al. patent were found to be very effective for their intended purpose as summarized above.
However, the rigid-flex printed circuit board formed according to the Dixon, et al. patent was found to be relatively expensive and time consuming to manufacture because of the additional steps of forming the (Kapton/adhesive) insulator layers or sheets and then laminating those combinations onto the flexible sections of the PCB while laminating a single prepreg layer over the rigid sections to form printed circuit board subassembly.